Sometimes sending emails can take longer than expected. This can cause a server timeout and wrong statistics in some cases. You’ll get a notification in this case:

Also if you’re on the cron.php page you get this:

If this is a temporary problem and the notification box vanishes by itself you can ignore it but should consider the following things if it happens regularly :

  • Increase your server’s memory limit
  • Decrease the emails sent at once at Settings => Newsletter => Delivery

You can also calculate your sending limits.